Hello,
If you want to have a good 3DOF challenge and complexity, I will suggest you to do some research on the Delta Robot:
https://en.wikipedia.org/wiki/Delta_robot
You can also easily find some 3D Model to print on website like Thingiverse:
https://www.thingiverse.com/
And a lot of DIY Kit exist on the market or are available on YouTube...event with LEGO :open_mouth:
Good Luck!
the complexity should be enough so that it displays how Mechatronics is implemented
I am using a the idea similar to a 3dof pneumatic arm and any further suggestions or improvements